1. Field of the Invention
The present invention relates to an image forming apparatus, such as a copying machine, a printer and a facsimile device. More particularly, the present invention relates to a sheet conveying path switching device for switching a sheet conveyance direction in such an image forming apparatus, and to a sheet conveying device that uses this sheet conveying path switching device.
2. Description of the Related Art
In order to downsize the whole image forming apparatus, it is desired to reduce the size of a sheet conveying device that conveys a recording sheet as a recording medium from sheet storage means such as a paper cassette or a paper feed tray. Also, in recent years, image forming apparatuses that support various sizes and types of sheets have become popular. In such image forming apparatuses, sheets of several sizes and types are previously stored in sheet storage means, and a type or size of sheet that is selected arbitrarily by a user or a sheet that is selected automatically by the image forming apparatus can be fed. For this reason, the sheet storage means fills a larger space in the image forming apparatus, thus there is a strong need of reducing the size of the sheet conveying device. From the standpoint of conveyance of sheets, the same thing is required in ADF documents as well.
Furthermore, recently, image forming apparatuses that are capable of selecting one-side printing or both-side printing of a sheet to form an image thereon have become popular. In an image forming apparatus capable of forming an image on both sides of a sheet, a sheet, one side of which has an image fixed, is sent out to a recording body loading section (sheet discharging tray) provided in the apparatus main body, or conveyed to a recording body reversing section (both-side conveying means) in order to form the image on the reverse side. Therefore, separating means for selectively switching the sheet conveyance direction is provided within the sheet conveying path. Even in the case of forming an image on one side, the conveying path needs to be switched when sheets onto which images have been fixed are sorted and discharged.
As described in, for example, Japanese Unexamined Patent Publication No. 2005-178954, there is adopted a sheet conveying device in which a simple and inexpensive separating arm is used as the separating means. In a conveying path within the sheet conveying device where a separating section is provided, the size of the conveyance direction is changed significantly, depending on the positional relation of the apparatus, and the space occupied by the conveying path is reduced in order to downsize the sheet conveying device. As a result, the conveying path is provided with a curvature section that has a predetermined curvature in order to change the conveyance direction of the conveying path continuously and smoothly, whereby the curvature radius of the curvature section can be set relatively small enough to convey a sheet of a fixed size, which is normally used in an image forming apparatus.
In the sheet conveying device having such a configuration as above, when conveying a highly rigid, firm sheet such as a cardboard, or a special type of sheet such as an envelope, the leading edge of the sheet P comes into contact with the wall surface of the curvature section due to the small curvature radius of the curvature section, whereby the friction resistance between the sheet and the wall surface increases. For this reason, there is a problem that the highly rigid sheet or special type of sheet cannot go forward along the conveying path formed in the curvature section, causing a paper jam or conveyance failure that inhibits stable conveyance operation. It is conceivable to use suction-type conveying means using air, in order to prevent such a problem described above. However, such conveying means requires a suction device, thus there is a disadvantage that the configuration of the sheet conveying device becomes complex and the size of the same increase, causing a cost increase.
In, for example, Japanese Patent Application No. 3257712, there is proposed a sheet conveying device that can securely feed sheets to each of the conveying paths intended to convey the sheets at small sheet intervals in response to the high speed of the sheet conveying device, the sheet conveying device being realized at low cost. In this sheet conveying device, as described hereinafter, by using a special switching belt, the conveying force can be applied to a sheet at a sheet-conveyance reversing section so that, even when conveying a firm sheet, smoother conveyance can be performed, compared to a fixed curving guide path having a small curvature radius. However, there is a problem in this device that a risk of a paper jam increases, as described hereinafter.
Technologies relating to the present invention are also disclosed in, e.g., Japanese Unexamined Patent Publication No. H10-129883 and Japanese Patent Application No. 3723067.